FIG. 4 shows a mechanism chassis 10 which is conventionally used in VCRs. The mechanism chassis 10 comprises a polygonal flat plate portion 11 for mounting on the surface thereof mechanisms such as a rotary head cylinder, a capstan, a pair of reel supports, and four side plate portions 12, 13, 14, 15 bent from the respective contour sides of the flat plate portion 11 toward the rear side of the plate portion 11. The flat plate portion 11 is formed with an opening 16 for mounting the rotary head cylinder, and loading grooves 17, 18 for mounting respective leading guide blocks for winding a magnetic tape around the rotary head cylinder.
To make the mechanism chassis 10, a blank having the contour of the chassis 10 of FIG. 4 as developed on a plane is cut out from a metal sheet having a thickness, for example, of 1.6 mm, and the blank is thereafter pressed to bend four side plate portions 12, 13, 14, 15 extending from a flat plate portion 11. A strain due to residual stress or the like will occur in the mechanism chassis 10 resulting from the press work, undulating the surface of the flat plate portion 11.
Accordingly, the bulging-out parts and the curving-in parts of the surface undulations of the flat plate portion 11 are reversely deformed, eventually using a leveler to correct the flatness of the flat plate portion 11. In this step, the mechanism chassis 10 has a sufficient thickness and therefore high rigidity, whereas the four side plate portions 12, 13, 14, 15, which are separated from one another, are free to deform at the separated portions B, so that the strain of the mechanism chassis 10 can be removed correctly by the eventual leveling work. As a result, the mechanism chassis 10 obtained has a high degree of flatness over the surface of the flat plate portion 11.
Mechanisms of simpler construction have been developed in recent years to provide compacted VCRs of reduced weight. It is required that the mechanism chassis 10 be prepared also from a metal sheet of the smallest possible thickness.
However, if the sheet thickness of the mechanism chassis 10 is reduced excessively a lower structural strength will result, readily entailing deformation due to the action of an external force and consequently leading to the malfunctioning of mechanisms on the chassis 10. Conventionally, therefore, there is a lower limit to the sheet thickness of the mechanism chassis 10, and difficulty is encountered in reducing the weight of the mechanism chassis 10.